Fibers derived from 5(6)-amino-2-(p-aminophenyl)benzimidazole (DAPBI), para-phenylenediamine (PPD-T) and terephthaloyl dichloride (TCl or T, also commonly referred to as terephthaloyl chloride) are known in the art. Such copolymers are the basis for a high strength fibers manufactured in Russia, for example, under the trade names Armos® and Rusar®. See, Russian Patent Application No. 2,045,586.
The two amines on DAPBI are very different in reactivity and positional factors. The amine shown to the right in the below structure (azole amine) is an order of magnitude more reactive than the amine to the left in the structure (benzyl amine).
As a result, DAPBI/PPD copolymers prepared by conventional a polymerization process in NMP/CaCl2 solvent system tend to have no control over the position of the monomer components. It is believed a factor in creating higher strength fibers from DAPBI/PPD copolymer is the arrangement of the comonomers along the polymer chain. In particular, controlling the distribution of PPD and DAPBI comonomers is believed to help prevent the formation of crystal solvates in sulfuric acid solution and help the alignment of polymer chains during the heat treatment of fibers made from the copolymer, resulting in fibers with better mechanical properties.
DAPBI/PPD-T copolymers prepared by conventional polymerization process (e.g. 35/65 split addition of TCl) in a NMP/CaCl2 solvent system also are known to contain sizeable amount of oligomeric blocks. It is well-known to those skilled in the art that these small molecular weight oligomeric species reduce the tenacity potential of the fiber made from these polymers. The oligomeric species serve as defects that cause premature breaks and hence lower tensile properties. It is therefore desirable to reduce the amount of these oligomeric species in the copolymer.